The present invention is generally directed to a print wire driving device for wire type dot or dot matrix printers.
In wire type dot printers, it is necessary for a number of print wires, and apparatus for driving these wires, to be assembled within a limited space. Japanese Patent Publication No. 58-35475 is typical of a configuration in which a plurality of rockable driving plates are radially arranged and a print wire is secured to the end of each of the driving plates. The driving plates form part of a magnetic circuit.
This conventional configuration requires that each driving plate be of at least some minimum thickness near its end so that a minimum cross-sectional area can be acted upon by magnetic flux of a given density in order to move the plates and thereby drive the wires. Such limitation in the reduction of the size of the driving plate results in a large head structure. Further, the driving plates have a great deal of inertia and therefore deteriorated response characteristics.
In these conventional configurations, the driving head is composed of a large number of small parts which must be positioned and secured to one another with machine screws. Production of such driving heads is labor intensive and not efficient. Further, a high degree of precision cannot generally be obtained, The driving plates are generally pivotally secured at their outer ends by a driving plate cap which pinches the plates from above. As a result of the outer ends serving as a support point, the driving plates move excessively in a direction perpendicular to the required axial rocking motion during operation. This adversely affects the printing action by increasing frictional forces or causing bending of the print wires. Further, in this type of configuration it is very difficult to assemble the driving plates, which are arranged in a plane, so that they are pivotally supported by the cap plate without displacement from their proper positions.
In order to increase the number of dots which form a character to improve the printing quality, a number of print heads are often stacked one upon another in the axial direction of the head assembly because only a limited number of print wire driving members can be assembled in a limited planar area. However, it is difficult to accurately maintain the desired positional relationship between the print wires from the axially stacked driving assemblies to the nose section of the printing head.
In the driving mechanism, electromagnets of the smallest possible size are arranged in a circle to attract and drive print wire driving magnetic segments, positioned in facing relationship to the electromagnets with a given required force. A magnetic pole surface of each core is generally positioned inside a coil in order to maximize the initial force of attraction. To achieve such a configuration, the magnetic pole surface of each core must be positioned close to the peripheral end section which pivotally supports the print wire driving segments. It is generally very difficult to machine these magnetic pole surfaces so that they are all positioned in the same plane. As a result, variations in the strength of attraction of the print wire driving segments often occur thereby resulting in deteriorated printing quality due to lack of uniform darkness in the printed character.
In conventional wire driving devices, current may be provided to the coils of the electromagnets by means of a flexible printed wiring cable. A conductive pattern formed on an end of the substrate corresponds to electrical connection pins electrically connected to the coils of the electromagnets. Solder is applied to electrically connect the pattern to the pins. However, due to the presence of a large number of pins, the conductive pattern is generally complex and each conductive path of the pattern is very narrow. This lowers the reliability of the soldering and decreases the current carrying capacity of the conductive pattern.
Generally the pins, which are soldered to the conductive pattern, are those of a coil bobbin. This requires that two metal pins be affixed to the bobbin, each serving as a terminal for the electromagnet coil. Each metal pin is press fitted into the substrate of the bobbin and the end of the winding is coiled around the pin which is then soldered to an island of the circuit pattern. This arrangement is labor intensive in that the metal pins must be prepared and then affixed to the bobbin. Further, after soldering to the conductors of the printed wiring cable, the ends of the pins projecting beyond the substrate thereof must be removed. In addition, a relatively long period of time is required to perform soldering due to the large heat capacity of the metal pins.
In conventional configurations, a supporting member for holding the print wire driving mechanism and for leading the print wires out to the nose of the print head is generally made of a synthetic resin due to the complexity of its shape. The rigidity of such a conventional supporting member is poor. Thus, precision of mounting is difficult to achieve even if a print wire guide member, attached to the nose or the print head, is mounted by the supporting member to a carriage. Heat which is generated inside the head assembly cannot effectively escape toward the carriage because of the low thermal conductivity of the synthetic resin.
In conventional configurations, the print wire guide member, attached at the nose of the print head, contains a number of guide holes bored in its surface which serve to guide a corresponding number of print wires and to align the wires in accordance with the size of a font to be printed. Each print wire is vibrated in the longitudinal direction of the guide holes by means of the electromagnetic driving mechanism. This causes a great deal of wear, enlarging the guide holes, thus reducing printing quality to an extent dependent upon the nature of the fit between a print wire and its corresponding guide hole.
Accordingly, there is a need for a print wire driving device having a configuration which results in improved positioning and maintenance of the position of the print wire driving segments, has an improved structure for connecting the electromagnetic coils to the flexible printed wiring cable, has a bobbin structure which is less costly and requires less labor to produce, and has supporting members and print wire guides which have close tolerances and are not subject to wear.